1. Field of the Invention
The present invention relates to the field of balloon dilatation. Specifically, the present invention relates to balloons for dilatation applications and a process for manufacturing the balloons.
2. Related Art
Surgical procedures employing balloons and medical devices incorporating those balloons (i.e. balloon catheters) are becoming more common and routine. These procedures, such as angioplasty procedures, are conducted when it becomes necessary to expand or open narrow or obstructed openings in blood vessels and other passageways in the body to increase the flow through the obstructed areas. For example, in the technique of Percutaneous Transluminal Coronary Angioplasty (PTCA), a dilatation balloon catheter is used to enlarge or open an occluded blood vessel which is partially restricted or obstructed due to the existence of a hardened stenosis or buildup within the vessel. This procedure requires that a balloon catheter be inserted into the patient's body and positioned within the vessel so that the balloon, when inflated, will dilate the site of the obstruction or stenosis so that the obstruction or stenosis is minimized, thereby resulting in increased blood flow through the vessel. Often, however, a stenosis requires treatment with multiple balloon inflations. Additionally, many times there are multiple stenoses within the same vessel or artery. Such conditions require that either the same dilatation balloon must be subjected to repeated inflations, or that multiple dilatation balloons must be used to treat an individual stenosis or the multiple stenoses within the same vessel or artery. Additionally, balloons and medical devices incorporating those balloons may also be used to administer drugs to patients.
Balloon catheters traditionally comprise a dilatation balloon at their distal end. Angioplasty balloons are currently produced by a combination of extrusion and stretch blow molding. The extrusion process is used to produce the balloon tubing, which essentially serves as a pre-form. This tubing is subsequently transferred to a stretch blow-molding machine capable of axially elongating the extruded tubing. U.S. Pat. No. 6,328,710 B1 to Wang et al., discloses such a process, in which tubing pre-form is extruded and blown to form a balloon. U.S. Pat. No. 6,210,364 B1; U.S. Pat. No. 6,283,939 B1 and U.S. Pat. No. 5,500,180, all to Anderson et al., disclose a process of blow-molding a balloon, in which a polymeric extrudate is simultaneously stretched in both radial and axial directions. Dilatation balloons are subsequently attached to a catheter shaft and wrapped down tightly on this shaft in order to achieve a low profile at the distal end of the catheter. The low profile serves to enhance the ability of a dilatation catheter to navigate narrow lesions.
The basic design of dilatation balloons has remained, essentially, unchanged since conception. The materials used in balloons for dilatation are primarily thermoplastics and thermoplastic elastomers such as polyesters and their block co-polymers, polyamides and their block co-polymers and polyurethane block co-polymers. U.S. Pat. No. 5,290,306 to Trotta et al., discloses balloons made from polyesterether and polyetheresteramide copolymers. U.S. Pat. No. 6,171,278 to Wang et al., discloses balloons made from polyether-polyamide copolymers. U.S. Pat. No. 6,210,364 B1; U.S. Pat. No. 6,283,939 B1 and U.S. Pat. No. 5,500,180, all to Anderson et al., disclose balloons made from polyurethane block copolymers.
Traditionally, the balloons available to physicians were classified as either “compliant” or “noncompliant.” This classification is based upon the operating characteristics of the individual balloon, which in turn depended upon the process used in forming the balloon, as well as the material used in the balloon forming process. Both types of balloons provide advantageous qualities, which were not available from the other.
A balloon that is classified as “noncompliant” is characterized by the balloon's inability to grow or expand appreciably beyond its rated or nominal diameter. “Noncompliant” balloons are referred to as having minimal distensibility. In balloons currently known in the art (e.g., polyamide block copolymers), this minimal distensibility results from the strength and rigidity of the molecular chains which make up the base polymer, as well as the orientation and structure of those chains resulting from the balloon formation process. The strength resulting from this highly oriented structure is so great that when the balloon is subjected to typical inflation or operating pressures (i.e., about 70 psi to over 200 psi), it will not be stressed beyond the failure point of the polymeric material.
A balloon, which is referred to as being “compliant”, is characterized by the balloon's ability to grow or expand beyond its nominal or rated diameter. In balloons previously known in the art (e.g., polyethylene, polyvinylchloride), the balloon's “compliant” nature or distensibility results from the chemical structure of the polymeric material used in the formation of the balloon, as well as the balloon forming process. These polymeric materials have a relatively low yield point. Thus, the inflation pressures used in dilation procedures are typically above the yield point of the materials used to form distensible balloons. A distensible or “compliant” balloon when inflated to normal operating pressures, which are greater than the polymer material's yield point, is subjected to stress sufficient to permanently realign the individual molecular chains of the polymeric material. The realignment of the individual polymer chains permits the balloon to expand beyond its nominal or rated diameter. However, since this realignment is permanent, the balloon will not follow its original stress-strain curve on the subsequent inflation-deflation cycles. Therefore, the balloon, upon subsequent inflations, will achieve diameters that are greater than the diameters that were originally obtained at any given pressure during the course of the balloon's initial inflation.
The yield point of a material is defined as the stress at which the individual molecular chains move in relation to one another such that when the pressure or stress is relieved there is permanent deformation of the structure. The modulus of a material, also known as the Young's modulus, is the stress per unit strain. A material, which exhibits the ability to follow the same stress-strain curve during the repeated application and relief of stress, is defined as being elastic and as having a high degree of elastic stress response.
Despite the use of high strength engineering polymers, access to highly occluded vessels and lesions in small vessels is still limited. Dilatation balloons currently available do not have a proper balance of competing properties. Balloons are needed that have low profile and are highly elastic, but also have high strengths and have high trackability to maneuver through tortuous vessels. While balloons made from polyethylene terephthalate (PET) can have lower profile than other balloons, such as polyamide copolymer balloons, the PET balloon is stiff, has a higher modulus, and therefore has inferior trackability. While balloons made from polyamide copolymers have better trackability than PET balloons due to their lower modulus, they have higher profiles thus limiting their application.
Furthermore, in attempting to produce low profile balloons by wrapping the balloon, the wrapping process often serves to reduce the burst strength of the balloon. An additional disadvantage is that dilatation balloons are not highly elastic. The initial low profile is only applicable to the first lesion that is dilated, as the balloon does not re-wrap tightly upon deflation. In the event that a patient has multiple lesions, a new catheter is used for each lesion thus adding to the cost and time of the procedure.
New dilatation balloon materials are needed that have the proper balance of these competing properties. Also, new processes are needed to produce dilatation balloons with the balanced properties. Dilatation balloons are needed that have low profile, high hoop strength, high-elasticity, high elastic recovery and high trackability.